If you don't mind horror, you should definitely check out Saya no Uta. Other short VNs I highly recommend are Planetarian and Narcissu. Be warned though, both are really sad. And Saya no Uta isn't exactly a mood lifter either. But they are definitely 3 excellent VNs
Regarding stuff that's less on the depressing side I would recommend Juniper's Knot and The Dandelion Girl. Both of these are really short though. Each only take slightly more than an hour to read. I also would recommend checking out Katawa Shoujo, as while it isn't exactly a "short" VN (it isn't a long one either), it's split up into parts. Act 1, which is the first part only takes like 3 or 4 hours to read. If you complete Act 1 and you're liking it you should continue with it, and if not, feel free to drop it.
Lastly, while I can't personally recommend it since I haven't read it myself (honestly, shame on me). Phenomeno is a short VN that a lot of people I know liked.
